Leslie Thomas contracted ulcerative colitis after taking part in a trial of the anti-inflammatory drug Vioxx

Hundreds of patients treated with a controversial painkiller could share millions in compensation after a pensioner scored a landmark victory against a drug giant.

Leslie Thomas, 75, was perfectly healthy and only took the anti-inflammatory drug Vioxx as part of an unpaid trial to test its effects on prostate cancer.

He suffered severe side effects including blood poisoning, fits and a collapsed colon.

The retired college lecturer from Ely, Cambridgeshire, was a 'virtual invalid' for nine months after he was diagnosed with an incurable intestinal disease. He won his case against manufacturers Merck last month.

The U.S. firm announced it would appeal but has now paid Mr Thomas £28,000 in an out-of-court settlement and his legal fees, which are understood to top £100,000.

Legal experts believe the case could open the floodgates for up to 450 other claimants in Britain.

Payouts could top £12 million if every claim is settled with similar levels of compensation. Legal costs could add millions to the bill for Merck.

More than 9,500 Britons will miss out, however, as they failed to lodge claims in the U.S. or UK in time.

Around 22,000 Americans have already been paid an average $250,000 since the drug was voluntarily withdrawn by the company in 2004 after it was linked with heart attacks and strokes.

Mr Thomas, who lives with his wife Norma, 69, yesterday said he had accepted the compensation to avoid going through appeal hearings.

'I'm not happy with the sum of money but I'm thrilled at the principle,' he said.

Speaking after his legal victory last month, he said: 'I've taken on a major company and won.

'Over the last few years I have been bullied by an unscrupulous drug company that tried to avoid all responsibility.'

It was in December 2003 that Mr Thomas, a fit grandfather of eight who enjoyed hill walking, responded to a letter from his GP and agreed to take part in a trial of the anti-inflammatory drug, also known as Rofecoxib.

It was already used by millions of people around the world, mainly older people with osteoarthritis, and he was assured he was unlikely to suffer side-effects.

But he suffered constant bloody diarrhoea for two months and was rushed to Addenbrooke's Hospital in Cambridge in June 2004 in a serious condition.

Doctors diagnosed him with ulcerative colitis - an incurable disease that causes inflammation of the lower intestine.

He has still not regained his full strength, nearly five years after his ordeal.

Mr Thomas, who began his fight for justice since July 2004, has not been entitled to legal aid to fight Merck but managed to fund the initial phase of his claim, costing £50,000, using a clause under his home insurance.

A firm of solicitors then took up his case on a no-win, no-fee basis and it was heard at Cambridge County Court.

During the trial, Merck claimed there was insufficient evidence to prove Vioxx had caused Mr Thomas's condition.

But Judge Patrick O'Brien ruled in the pensioner's favour, saying: 'I am satisfied that, on the balance of probability, Vioxx ingested during the drug trial was a significant cause of Mr Thomas' ulcerative colitis.'

If it were later proven the drug had not caused the condition, he added, he would find it had exacerbated the condition.

But Gerard Dervan, a solicitor representing more than 100 of the affected Britons, said: 'This is a huge development. It removed a big barrier and I hope that it will herald the start of more cases.

'The main risk faced by potential claimants is the appeal process. It was feared that any claimant who won their case in a county court would find themselves fighting appeal after appeal. This would have been an enormously costly process beyond our claimants' means.

'Now, we have seen this massive firm roll over and drop the fight. The possibility of a real victory is in our sights. Merck wheeled in the big guns to fight this case and lost.'

Solicitor Nikki Powell, of London class action specialists law firm Edwin Coe, said: 'This is a potentially important test case on which future claimants will be able to rely.'

Last night, a Merck spokesman said: 'This case is unique and has no significance for any other claim pending in the UK.'